The present invention relates to a method of adjusting a heat-displacing T-die for sheet formation and film formation, and more particularly to a method of adjusting a heat-displacing T-die to permit convenient and proper profile control of sheets and films by cascade control to a lip adjusting amount of the T-die in a virtual temperature adjusting loop for estimation and operation on the basis of control output data of a heat-displacing actuator and a thickness feed back control loop calculated from data of thickness of formed products.
In general, a heat-displacing T-die having a structure as shown in FIG. 1 is used for sheet formation and film formation. The heat-displacing T-die has a body 10. A T-die lip 14 is formed as a part of the body 10 of the heat-displacing T-die. The T-die lip 14 has a slit gap 12 for extrusion molding of a molten resin into the shape of a sheet. A large number of die bolts 16 are further provided which extend in a direction along which the slit gap 12 is formed, so that the die bolts 16 are made into contact with the T-die lip 14. Each of the die bolts 16 is provided with a temperature-adjustable heater 18 which is electrically connected to a cable 20 for supplying a controlled current to the heater 18. Another heater 22 is provided on a top surface of the body 10 of the heat-displacing T-die for temperature control of the body 10. Inside of the T-die lip 14, a flexible necked down portion 14a is formed in the body 10.
The slit gap 12 of the T-die lip 14 is adjusted by thermal expansion of the die bolts 16 having received a heat from the heater 18.
There are known in the art a heat direct-acting type and a heat reverse-acting type.
As illustrated in FIG. 2, the heat-displacing T-die of the heat direct-acting type has the following structure. A heater holder 18 is provided for holding the die bolt 16. The heater holder 18 holes a first end portion 16a of the die bolt 16 near the T-die lip 14, so that a temperature rising of the heater 18 causes an expansion of the die bolt 16 toward the T-die lip 14 whereby the first end portion 16a of the die bolt 16 presses down the T-die lip 14. As a result, the slit gap 12 of the T-die lip 14 is narrowed.
As illustrated in FIG. 3, the heat-displacing T-die of the heat reverse-acting type has the following structure. A heater holder 18 is provided for holding the die bolt 16. The heater holder 18 holds a second end portion 16b of the die bolt 16 opposite to the first end portion 16a near the T-die lip 14, so that a temperature rising of the heater 18 causes an expansion of the die bolt 16 in a direction opposite to the direction toward the T-die lip 14 whereby the second end portion 16b of the die bolt 16 is withdrawn from the T-die lip 14. As a result, the slit gap 12 of the T-die lip 14 is widened.
In the prior art, as the method of adjusting the heat-displacing T-die, a feed back system of FIG. 4 and a cascade system of FIG. 5 have been known.
In accordance with the feed back system of FIG. 4, a target profile is transmitted through a profile control unit 30, a die bolt heater control unit 32, and a die bolt-lip system 34 to a forming processor 36 for profile control of a sheet. The result of the profile control is then processed in a profile processor 38 so as to be fed-back to the profile control unit 30. Those feed back control and operations are carried out at a constant period. A power to be supplied to the heater of the die bolt is renewed step-like. As a result, a thickness of the sheet at a correspondence position is exponentially increased and then reaches an equilibrium state, during which it takes a time of a few times as long as a time constant of the heater. The above variation can be detected after an unnecessary time L has been passed. For this reason, a vibration in the shape of a saw-tooth is likely to appear in the profile due to overshoot. Since the influence by external disturbance can be detected only by a thickness gauge, a correct operation to the externally disturbed D1 to D4 is late thereby making it difficult to improve the accuracy of the adjustment.
FIG. 5 is illustrative of the cascade system wherein the target profile is transmitted through a profile control section 40, a temperature control section 41, a die bolt heater control unit 42 and a die bolt-lip system 44 to a forming processor 46 for profile control of the sheet. In this case, the temperature of the heater is detected, so that the detected temperature value is then fed back to the temperature control unit 41. The result of the profile control made by the forming process is then processed by a profile processing unit 48 for feeding the same back to the profile control unit 40 for performing a cascade control so that the temperature to be set for each die bolt is renewed by step like and a power to be supplied to the heater is adjusted.
It is, for example, disclosed in the Japanese Patent Publication No. 1-22140 that processing is made for the mutual thermal interference between adjacent die bolts to calculate the temperature having been newly set for each die bolt so that an initial die bolt temperature having previously been set, which provides no temperature influence to the material, is compared to an average of all of the newly set die bolt temperature calculated so that the average is adjusted to correspond to the initial die bolt temperature whereby a heat-displacing T-die slit gap is adjusted in the vicinity of an optimum temperature for forming the material.
In the cascade control system, a slave loop for the die bolt temperature is provided in a master loop for the profile as illustrated in FIG. 5, for which reason the die bolt temperature having been set is renewed step like by an instruction of the master loop whereby PID operation is effected for quick correction. D1 and D2 in the external disturbances are almost completely processed in the slave loop, resulting in remarkable improvements in responsibility and stability.
In the above conventional feed back system, the temperature of the heater of the heat-displacing T-die is not detected and a control to the heater is decided directly from the thickness deviation, for which reason the following advantages and disadvantages are caused.
It is advantageous that since no temperature sensor is provided, the heater has a simple structure and the control unit with one loop is also simple.
It is, however, disadvantageous that it takes a long time for movement to the thickness gauge of the sheet extruded from the heat-displacing T-die. The displacement of the T-die lip is caused by thermal expansion of contraction of the die bolt due to a temperature variation of the heater. Even if the power to be supplied to the heater is kept constant, then the temperature is varied.
In the above cascade system, in order to improve the disadvantages of the feed back system, a temperature sensor is provided for the die bolt provided with the heater for temperature control of individual die bolt. This cascade system has the following advantages and disadvantages.
It is advantageous that since in order to detect and manage the temperature of the heater, variation of the heat-displacing T-die slit is predictable directly from the detected value of the die bolt temperature, a temperature control loop can be set in a constant time regardless of an unnecessary time from the heat-displacing T-die from the thickness gauge.
It is, however, disadvantageous that a temperature sensor is provided for each die bolt whereby the structure is complicated and the wiring of leads between to the heater and the sensor is also complicated. As a result, the manufacturing cost thereof is increased.
In the above circumstances, it had been required to develop a novel method of adjusting a heat-displacing T-die in a cascade system of a temperature control loop and a thickness feed back control loop free from the above disadvantages.